SEEKING SOLUTIONS: Floodwaters inundated the area near the Aquarium T Stop on March 2, a much different scene yesterday as pedestrians mill about. A study says building a giant wall will be too costly and inefficient.

Building a giant wall across Boston Harbor to prevent devastating floods like the ones that swamped the city this winter would take 30 years, cost up to $12 billion and be no more effective than land-based prevention, according to a study that recommends spending $2 billion instead on neighborhood-focused efforts.

And while harborside activists agreed the giant wall wouldn’t make Boston dry again, they said those local efforts aren’t moving fast enough to prevent further flooding.

The report is set to be released today by University of Massachusetts Boston’s Sustainable Solutions Lab and the Green Ribbon Commission — a group of business leaders and think tanks focusing on climate change.

It rejects the idea of using massive harbor barriers with closeable gates to prevent flooding due to increased sea level rise. Heavy storms combined with high tides left large parts of Boston underwater last winter, and scientists predict the average sea level could rise by up to 7 feet by 2100.

The report looked at three possible barriers — a series of dikes between Swampscott and Cohasset, an outer harbor wall from Winthrop to Hull and an inner harbor wall from Logan International Airport to the Seaport. The authors determined none of them would be effective.

The large dike system was rejected early on because it would cause environmental damage and affect shipping routes, according to Paul Kirshen, the UMass professor of climate adaptation who led the study.

And while the inner and outer walls would not have negative environmental effects, they would still affect shipping and come with hefty construction price tags — between $6.5 billion and $8.7 billion for the inner barrier and between $8 billion and $11.8 billion for the outer one. And even if the planning process began today, the walls would not be ready until 2050 — and their low cost-effectiveness ratio would make them unlikely candidates for federal funding, Kirshen said.

In addition, Kirshen said the barriers would likely wind up closing their gates more often than planned, leading to wear and tear that would reduce the structure’s lifespan.

“It’s putting all our eggs in one basket,” Kirshen said, saying the better option was sticking to local measures outlined in the city’s Climate Ready Boston plan. “Shore-based solutions can provide equivalent protection to an off-shore barrier and do it at a lot less cost, a lot more flexibility and a lot more co-benefits than the barrier system.”

East Boston activists agreed, but said they’re still waiting on some of those neighborhood plans. Magdalena Ayed, a member of the Harborkeepers coastal resiliency group, said residents are waiting for more information on a planned floodwall for the East Boston Greenway that has been proposed but not funded.

“The whole conversation about a barrier is kind of sidetracking some of the solutions we need to focus on on the community level,” Ayed said. “I support Climate Ready Boston 100 percent, but how many initiatives are you really implementing?”

City officials said the floodwall would be installed by the end of the year.